1. The Field of the Invention
The present invention relates to the field of microfluidic circuitry for biochemical processes or reactions. It relates more specifically to the sensing and regulation of the pressure and movement of fluids through the microfluidic circuitry.
2. Background of the Invention
Microfluidics involves the manipulation of small volumes of fluid in structures. having microdimensions and formed in a substrate of silicon, plastic, glass, or the like. Microfluidic devices often contain multiple channels or wells and are used for chemical or biological processing and analysis of liquid samples. These channels and wells in microfluidic devices are known as microfluidic circuits. If a microfluidic circuit has any degree of complexity, a method is needed, either active or passive, for controlling the flow of fluid through the circuit.
Active methods of fluid control include the use of mechanical valves and the application of electrical or magnetic fields to influence the movement of fluid (or particles in the fluid) within the microfluidic circuit. However, incorporating mechanical valves into a microfluidic circuit can make it complex and expensive. Electromagnetic field methods may require complex interfacing and possibly high electrical voltages.
Passive methods of fluid control usually involve the manipulation of capillary forces to stop or drive fluid movement. Passive methods may not be possible if the fluid is or contains a large concentration of solvents, surfactants, lipids or aliphatic compounds, because they may reduce the surface tension of the fluid, which reduces the capillary forces.
It has been recognized that if passive methods cannot be used and the characteristics of field methods are not desirable, it would be beneficial if the complex valving mechanisms used for active fluid control, could be moved off of the substrate. In this manner the microfluidic circuit substrate could be made as inexpensively as possible by transferring the complex and expensive components into a permanent fixture, thus allowing the microfluidic substrate portion to be inexpensive and disposable.
One example of such a design is the use of pneumatic actuation performed with external pumps and valves. Another example is the use of external actuators for moving diaphragm membrane valves that are within the circuit. These two examples, however, still require complex structures within the substrate such as the hydrophobic air ducts or flexible membranes. These attempts at moving the complex mechanical structures off the substrate are therefore more complicated than may be desirous.
Another exemplary method for regulating the movement of fluids through a microfluidic circuit is to combine passive and active control methods to utilize an air vent (or air duct) in support of a capillary barrier in cooperation with a capillary stop junction. In this method, the fluid flows through a capillary channel and is primarily controlled by a capillary stop junction. The reliability of the capillary stop junction is increased by the addition of an air vent. The fluid is drawn through the microfluidic circuit by positive capillary forces, such as aqueous fluids being drawn by capillarity through a hydrophilic channel. When the air vent is closed, the air vent supports the capillary barrier at the capillary stop junction to control the advancement of fluid through the microfluidic circuit. Because the air vent supports a capillary barrier, this method of fluid control will not function independent of a capillary junction.
These attempts to effectively control the fluid flow within a microfluidic circuit rely primarily on expensive mechanical devices within the substrate or on capillary forces to provide flow barriers. It would be a significant improvement in the art to provide a microfluidic circuit that is capable of controlling fluid flow without the use of capillary forces. It would also be a significant advancement in the art to provide a substrate for a microfluidic circuit that does not incorporate any complex or expensive parts, thus allowing the substrate to be inexpensive and disposable.
The present invention is directed towards a microfluidic circuit in which fluid flow through the circuit is regulated by external valves that control the displacement of air within the circuit. Air displacement ducts are connected to external valves. If the valves are open fluid advances into a circuit driven by positive hydrostatic pressure, such as that generated by an external syringe pump. If the valves are closed, the advancing fluid pushes against a closed air column, or a pneumatic pressure barrier, which, under normal operating parameters, stops the further advancement of the fluid. A pneumatic pressure barrier can be used not only to stop fluid advancement in a circuit, but also to divert fluid flow from a blocked channel into an adjacent, open channel. In this manner the fluid can be controlled as it advances through a possibly complex, highly multiplexed system. This control is performed by the use of external valves and pumps that do not need to be incorporated into the microfluidic substrate. Once fluid has reached an air escape duct it is prevented from entering the duct by the use of capillary barriers or other passive valves at the junction between the fluid channel and the air duct, or by blockage of the duct by a swellable material such as a hydrdogel. Entry of fluid into the duct can also be blocked by closing the external air duct valve at the appropriate time in the fluid manipulation or by placing a fixed volume air bladder at the outlet of the air duct, either of which serves to establish a closed air column against which the fluid cannot advance.
The microfluidic circuit is constructed in a substrate. The substrate has at least one channel for fluid flow and at least one air duct in communication with each channel. There is at least one stopping point in the circuit where the fluid is at least temporarily stopped. The fluid is stopped at these stopping points by a controllable pneumatic pressure barrier in the circuit. Conveniently, the air ducts in communication with the channel are configured to control the pneumatic pressure barrier, and there is an air duct in communication with each of the stopping points within the circuit.
The pneumatic pressure barrier that controls the advancement of the fluid through the circuit is formed by the fluid entering the circuit and compressing the air within the channel and the air ducts. Because there is no outlet for the compressed air, it prevents the advancement of the fluid. The fluid is subsequently allowed to advance by opening a downstream air duct that allows the compressed air to escape. The downstream air duct may be opened to the atmosphere or it may be opened to a fixed volume expansion bladder. Preferably, the air ducts are configured to close when the fluid reaches the stopping point with which each air duct is associated, such that the fluid is stopped in its progress through the circuit and is also prevented from advancing too far into the air duct.
To facilitate the proper opening and closing of the air ducts, the microfluidic circuit may further comprise a sensor that determines the location of the fluid within the circuit and signals for the closure of air ducts such that the fluid flow is at least temporarily stopped at a stopping point. The sensor may be an optical sensor, and it may be located in a top plate that overlies the fluid channels. Alternatively, the sensor may be a fluid pressure sensor located at the fluid inlet to measure the backpressure on the fluid or an air pressure or flow sensor located on an air duct to measure the pressure or flow within the air duct.
The present invention is also directed towards a method of controlling fluid flow within a microfluidic circuit utilizing the microfluidic circuit described and pneumatic pressure barriers. The movement of fluid together with the opening and closing of air ducts, as well as injection of air, are used to generate pneumatic pressure barriers. For example, it is difficult or impossible to introduce fluid into a tube that is only open on one end. Similarly, it is also difficult to introduce fluid into a tube with one end open and one end connected to a valve that is closed. If the valve were opened, air could escape at the same time as fluid enters the tube. Hence, the valve controls whether fluid flows within the tube. In addition to flow control, this invention also discloses methods and devices for connecting the air ducts with the flow channels. Various methods of closing the air ducts after they have been opened are also discussed. The use of the term xe2x80x9cairxe2x80x9d in this document is simply descriptive, and is meant to include any gas or gaseous phase regardless of composition.